Wednesday, 9 January 2008

Eye liner

Make a pinhole in a piece of cardboard. Bring your eye close to it and look through the pinhole as you rotate the card. You will see the network of your retinal capillaries against the background of a cloudy sky. How does this happen?

I looked at this phenomena some 14 years ago. It doesn?t fit any known physical 'attributes' like diffraction etc. I called it the 'blue spot phenomena' and set up some quick experiments. It has nothing to do with the eye, and I wrote a report on it. It has not been published, however I can release it to any interested party. My contact is Arturo.Excalibur@talktalk.net.Note the effect can be seen in any 'spot' source of light, like LED on computer keyboards. I have also photographed it; not too clever, but it can be done.

The capillaries are in an area where the light is uncollimated (non-parallel) and therefore they do not normally cast a shadow. The pinhole ensures that the light rays are collimated by restricting the incoming light to a narrow range of angles. The capillaries can then cast a shadow on the retina and can be seen. Imagine the shadow cast by chicken wire being washed out when the sun is diffused by light cloud. The pinhole behaves like the unobstructed sun and ensures sharp shadows. The eye is very insensitive to stationary images and the image quickly dissolves away if the pinhole is held steady. Moving it in a circle moves the shadow and ensures that you continue to see the image. The rapid flickering (saccades) of the eye muscles achieve the same purpose in normal vision; even when we focus on a fixed point.

I have noticed a similar effect especially when lying on my back looking up at a clear blue sky, or sometimes at any wide, uniform field of light colour. The image I get looks like corpuscles pulsing along fixed paths. I had assumed they were blood corpuscles moving in capillaries. Medical people I have spoken to dismissed this idea - but they have not seen what I have seen!

According to Murrillo-Lopez et al (2004; Investigative Opthalmology & Visual Science, 35(8): 3319), this phenomenon was first observed by Steinbuch in 1813, and Purkinje in 1815, hence their name Purkinje Vessel Shadows. It was first explained by von Helmholtz in 1924.

As previously posted, a bright light source close to the eye leads to the retinal blood vessels casting a shadow on the photoreceptor layer below. If the light sources stays still, the eye quickly adapts and the shadows are not perceived.

The movement of tiny right dots along fixed lines that you can see particularly against a blue sky is the Blue field entoptic phenomenon, or Sheerer's phenomenon. The dots are white blood cells travelling along retinal capillaries, which are less common than red blod cells. The eye compensates for red blood cells, so you don't see them, but occasional white blood cells can be noticed (Sinclair et al, 1989, Inv Opthal & Vis Sci, 30: 668).

I was once in a dark room, leaning my head against a window which had raindrops on the other side of it, and in one of the droplets I was amazed to see the surface of the inside of my eyeball in incredible detail. It was very much like N R Patersons description. There's never been any doubt in my mind that it was actually the inside of my eye, it was so vivid. Fascinating stuff! Chris.

This is a fascinating phenomenon known as Purkinje shadows, after the Czech physiologist and neuroanatomist Jan Evangelista Purkinje. It also illustrates an excellent argument against intelligent design.

In the human eye, light passes through all the nerve fibres and blood vessels before reaching the photoreceptors. This curious arrangement means that the blood vessels cast shadows, and it explains why the capillaries can be seen if you look through a moving pinhole. Surely a master creator wouldn't have made a mistake like that. After all, the squid eye is designed the other way around, which raises the possibility that the mythical intelligent designer of life considers cephalopods a higher form of life than humans.

The reason we don't normally see the shadows of the blood vessel is because the human eye is incapable of registering a stationary image. We can see things that don't move, such as statues or doors, only because our eyes are continually making tiny movements which ensure that their image jiggles across our retina. Because the blood vessels are part of the eye they move with it and usually remain invisible. Using sophisticated eye-tracking equipment it is possible to completely stabilise a retinal image. When this happens the image disappears, a phenomenon called Troxler's fading. If our eyes were completely still we'd be almost blind. Intelligent design, huh?

Moving a pinhole across the pupil changes the direction of light reaching the back of the eye, which has the effect of "moving" the capillaries relative to the retina, making them visible. An even better way to see the blood vessels in your own eye is to put a small bright pen torch near the white part of the eye (while being careful not to poke yourself).

What you are describing sounds like the blue field entoptic phenomenon. Red blood cells are plentiful in the narrow retinal capillaries and the brain has to attempt to edit them out to avoid dark squiggly lines in the image.

Occasionally a white blood cell, which are much less prevalent than the red ones, will pass through the same capillary and create a "void" where light can pass through more easily. These "voids" will appear as a faint bright spots travelling along squiggly lines as a result of the brain's attempt to edit something out which is not there.

Regarding the blue field entoptic phenomenon, if I look out of the window on an overcast day, and then look at a dark area (a stained fence opposite) it looks like it is raining lightly. I'm so relieved to get an explanation for this!

I've seen red blood cells (definity round,bumped red cells) ooze through the capillaries over my retinas, usueally looking up at a uniform blue sky or other flat, bright ( but not dazzling) field of view. Usually your brain/eye compensates but sometime you can make it out.

I've just done something similar - I looked at the main light in my room, with my head tilted back, and rested my eyelids to the point at which the eyelashes were obscuring my field of vision. What I saw, at the right angle of head inclination, was a strange structure in my left eye. It almost looked bacterial, like a large, suspended bacterium in my eye. I was pretty freaked out by it, as it is only in my left eye and clearly not a structure that's meant to be there. I wonder if this is some kind of structure (/organism?) that is somewhere between the cornea and the photoreceptors of the retina, thus something that casts a shadow??

These are interesting answers but the specks of white light against the sky when looked at for sometime move in spiral motions.This has been observed,by several of my colleagues and my-self.Try it for your-self, please let me know as for you it may be a different matter.

I don't think the argument against a master creator given by Jan Evangelista Purkinje is very convincing. It omits a basic principle of physics /chemistry,which is everything is made of atoms which are either positively or negatively charged. Anything carrying a charge vibrates due to the constant re-appearing and disappearing electrons and other effects. So in essence everything vibrates. and nothing is static, so this would point to it being a rather brilliant bit of design and perhaps indicating some thought had gone into it. Hence not totally ruling out intelligent design.

I have seen these capillaries very clearly at the optometrist when she shines a bright light up close to my pupil, presumably to have a look inside herself, it is fascinating - makes it hard to follow her instructions to look up, sideways etc, as i just want to look inside my own eye too !

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